144 EIGHTH REPORT 1838. 



and pas? in nearly a straight line from the intei-nal to the external sur- 

 face of the tooth : the pulp-cavity extends about half-way through the 

 body of the tooth, and has a narrow elliptic transverse section ; it be- 

 comes gradually smaller at the base of the tooth, and there branches 

 out into several processes, which are continued into the cylindrical 

 processes of the dental substance, which are iuibedded, like so many 

 piles, in the coarse osseous texture of the jaw. It is this peculiar mode 

 of fixation of the tooth to the jaw-bone that would serve at once to 

 distinguish the tooth of Holoptt/chus from that of any saurian or niam- 

 miferous species which it miglit resemble in external form. 



In not any of the teetli of fishes above described was there an- ex- 

 ternal covering of enamel, presenting the characteristic transversely- 

 striated prismatic crystalline structui'e which distinguishes the enamel 

 of the higher Vertebrata. In all cases, Avhere structure could be de- 

 tected in the dense exterior layer representing the enamel, it presented 

 the organized tubular character, differing from the subjacent ivory 

 only in the more minute size of the tubes. 



Of the teeth of reptiles, Prof. Owen described those of several genera, 

 recent and fossil. In the S/iarp-nosed Alliffcitor (Crocodihis aculus), 

 the exposed part of the tooth is covered ^vith true enamel, and that part 

 which is lodged in the socket is coated with a layer of ccsmentum. The 

 tubuli are very fine, not exceeding at the widest part y^ijyth of a line. 

 With a low magnifying power they appear to radiate in straight lines 

 from the cavitas pulpi to the superficies of the tooth, proceeding at 

 right angles to that surface : under a higher power, they are seen to 

 be slightly undulating, and to have interspaces equal to five times their 

 own diameters. The main tubes begin to divide soon after their origin, 

 and the branches diverge from each other ; these send off numerous 

 finer ramuli, which are generally turned towards the root: these ter- 

 minate or dilate, in many places, into calcigerous cells, which form 

 numerous layers, generally arranged parallel Avith the contour of the 

 cavity of the pulp, and most numerous at the circumference of the 

 ivorj'. It is to these layers of calcigerous cells, and to the parallel cur- 

 vatures of the tubes, that the apparent laminated structure is seen to 

 be due, when sections of these teeth are examined with a low magnifying 

 powei'. A thin membrane lines the cavity of the jjulp of even the oldest 

 teeth. 



The fossil teeth of the extinct Reptiles reveal an equally complicated 

 structure. The fang of the fluted teeth of the Ichthi/osmirus is covered 

 with a thick layer of camentum, which fills the interstices of the 

 grooves. The tubuli of the ivory-constituent are extremely minute; 

 they resemble in their arrangement and ramification those of the cro- 

 codile, but the undulations are more numerous and more marked. 



In ih.e Iguanodon, the ivory is composed of close- set tubes, radiating 

 in a wavy course from the cavitas ptdpi to the superficies : each tube 

 is also minutely undulating. They are coarser than those of the Ich- 

 thyosaurus ; and the ivory further differs in the presence of large me- 

 dullary canals, which are seen here and there radiating from the cavity 

 of the pulp, and traversing the dense ivory. 



